Add item_stack_limits to the conversion test's MobType

Main gave MobType an item_stack_limits field after this branch was cut.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client/src/game/zombie_villagers.rs

@@ -310,6 +310,7 @@ mod tests {
                 knockback_resistance: 0.0,
                 jump_strength: 0.42,
                 attributes: Arc::default(),
+                item_stack_limits: Arc::default(),
                 gravity: 0.08,
                 safe_fall_distance: 3.0,
                 fall_damage_multiplier: 1.0,